1. Field of the Invention
The present invention relates to a storage system for executing a remote copy operation, and more particularly to a storage system for specifying an object of remote copy in units of a file or a directory instead of in units of a volume.
2. Description of the Related Art
Conventionally, there is a technology which is referred to as remote copy for copying data between storage systems.
The remote copy is dual writing without an intermediation of a host computer among a plurality of storage systems physically apart from each other. In the remote copy, storage systems located in a first site and a second site are connected to each other through a private line or a public line. Additionally, a logical volume having the same capacity as a logical volume to be copied on the storage system at the first site (hereinafter, referred to as a copy source logical volume) is formed as a logical volume (hereinafter, referred to as a copy destination logical volume) paired with the logical volume to be copied on the storage system at the first site onto the storage system at the second site. Then, data in the first site copy source logical volume is copied to the second site copy destination logical volume. If the data in the first site copy source logical volume is updated by the host computer, the updated data is transferred to the second site storage system and written into the copy destination logical volume. In this manner, the duplicate condition of the logical volume is always maintained at the first site and the second site in the remote copy technology.
Therefore, even if the storage system at the first site is unusable due to a natural disaster such as an earthquake or a flood or human disaster such as a fire or terrorism, the logical volume on the storage system at the second site can be used to restart operations quickly.
Prior arts related to the remote copy are disclosed in U.S. Pat. No. 5,742,792 or in EMC Symmetrix Remote Data Facility (EMC SRDF) L523.7-J 11/00.1.
A network attached storage (NAS) is a storage appliance connected to a network and used for processing file level access requests. Unlike a block level storage device (SCSI-connected hard disk, etc.), the NAS has a file system such as the network filesystem (NFS) and is capable of processing access requests to a file or a directory.
A file server is based on a concept similar to the NAS. The file server is connected to a network and used for processing file access requests in the same manner as for the NAS. The NAS differs from the file server in a respect that the NAS is a storage as an appliance specialized in input-output services of files and therefore it is useful to construct a robust system resistant to system down in comparison with a normal file server, in compensation for an omission of processing completely unrelated to the input-output services.
Regarding the NAS, there is a description in “Nikkei Electronics; the 2000.11.6 (No. 782) issue” (P. 54, 55) published by Nikkei Business Publications, Inc.
Regarding the NFS and the NFS server, there is a detailed description in “4.4 BSD Operating System” (P. 311 to 336) by Marshall Kirk McKusick, et al. published by Addison Wesley.
In the foregoing prior art, however, the remote copy is executed in units of a volume and therefore objects of the remote copy includes minor files, which are not important and do not need to be duplicated, stored in the volume. As a result, an amount of data to be copied increases, thereby increasing the time for the copy.